In the adipocytes of mammals, white adipocytes, which store energy as triglycerides, and brown adipocytes, which generate heat by utilizing triglycerides to thereby contribute to the control of body temperature, are present. Hibernators and rodents, which have developed brown adipose tissues, can live by adjusting themselves to even a cold environment at about 5° C. In the case of human, brown adipose tissues are observed in the peripheries of the shoulder blades in a newborn infant, but the brown adipocytes decrease in accordance with growth.
On the other hand, brown adipocytes consume excess energy as heat by producing heat, and thus have attracted interests in relation to obesity and diabetes mellitus. Actually, significant development of obesity was observed in a transgenic mouse from which brown adipocytes had been ablated genetically (Non-patent Document 1). Furthermore, the present inventors generated a mouse with deficiency of mitochondria uncoupling protein (UCP1; uncoupling protein 1), which is a major molecule for the heat generation in brown adipocytes (Non-patent Document 2), and reported that this mouse developed diet-induced obesity with aging (Non-patent Document 3). It has been considered that, unlike experimental animals such as mouse, human brown adipocytes disappear with aging, and thus have substantial little roles in human. However, it was recently clarified that brown adipocytes are also present in adult humans, although the amount thereof is small, and the existing amount thereof inversely relates to a degree of obesity and a blood glucose level (Non-patent Document 4), and thus brown adipocytes have gained a great deal of attention since the induction of differentiation of brown adipocytes would be useful for the prevention and treatment of obesity and metabolic syndrome (Non-patent Document 5). On the other hand, with respect to transcriptional regulators that control the differentiation of brown adipocytes, it has been clarified that PGC1α (Non-patent Document 6) and PRDM16 (Non-patent Document 7) and the like play important roles.